1. Field of the Invention
The present invention relates to a method for driving a gas discharge display panel.
More particularly, the invention relates to a memory type gas discharge display panel. In this method a sustaining pulse is normally applied between display anodes and cathodes of the device. The sustaining pulse discharge is repeated during a term starting from an application of write-in pulse until an application of an erasing pulse. The time of the sustaining pulse from its rise-up until the establishment of the sustaining voltage is selected to be 150 ns to 500 ns or by arranging the waveform of the sustaining pulse not to increase in one steep slope but to increase stepwisely so that the problem of decreasing margin of the sustaining pulse in a large size display panel due to an increase of inductance in the electrodes and interelectrode capacitance can be solved.
2. Prior Art Statement
A method for improving luminous intensity of a gas discharge display panel by providing a memory function in a gas discharge display panel, which is so-called as a "memory driving system" had been patented for the applicant under Japanese Patent No. 1,486,701 with a title "A method for driving a gas discharge display panel". One embodiment of the discharge display panel using the driving method of this patent is shown briefly in FIG. 11.
Further, FIG. 12 is one embodiment of voltage waveform of driving voltage in a recent prior technique (Japanese Patent Application No. 1-272,919 by the present applicant entitled as "Method for driving gas discharge display panel"). The operating principle of said "pulse memory driving" will be explained briefly hereinafter.
A constant period sustaining pulse SP is normally applied to the display electrode D.sub.j periodically. The amplitude V.sub.sp and the pulse width T.sub.p of this sustaining pulse SP are previously selected to have such a value that a pulse discharge started by write-in pulse WP can be sustained even after the termination of the write-in pulse. Scanning pulses SKP are applied successively from first row cathodes. In an auxiliary cell AC.sub.ij, an auxiliary discharge is ignited at an auxiliary cathode A.sub.j. At a display cell DC.sub.i(2j-1), a write-in discharge is started together with a write-in pulse being applied to a corresponding display anode at a substantially same timing with the scanning pulse SKP. The auxiliary cell AC.sub.ij and the display cell DC.sub.i(2i-1) or DC.sub.i(2j) are coupled by ionization through microscopic space. The write-in discharge is started very quickly to be full discharge condition by the aid of the auxiliary discharge. In order to stop the sustaining discharge at a display cell, an erasing pulse ERS is to be applied to a corresponding cathode to stop the sustaining pulse discharge once or more.
When displaying a picture having half tone scene, like a television picture, it is necessary to shorten the write-in period of one row to about 4 .mu.s (in case of FIG. 12, this period is equal with the period of the sustaining pulse (T.sub.SP). In order to effect stable and high speed write-in, the pulse width of the access scanning pulse (SKP) requires a length of at least 2 .mu.s. As can be seen from the same diagram, the pulse width T.sub.p of the sustaining pulse is made at the most 1.7 .mu.s.
When driving a panel by using the conventional pulse waveform as shown in FIG. 12, a stable memory operation is possible only for a small size panel and only when a wide memory margin had been kept. However, when the panel size becomes large or a case of high discharge voltage and hence the pulse voltage is needed to increase, a stable memory operation becomes difficult.
In order to solve this problem, the inventors had examined the actual phenomena of discharge in detail and found the following.
(1) At some particular regions, erroneous discharge tends to be produced. By changing the driving side of electrodes, such regions appeared in a symmetrical position. PA1 (2) The built up of wave front of light of the sustaining pulse discharge in the above region was substantially speedier compared with that of other regions. PA1 (V.sub.SP)min is a minimum voltage to keep the sustaining pulse discharge PA1 (V.sub.SP)max is a maximum voltage which can be applied to the panel in which non-accessed cells will not cause erroneous discharge.
The above two points may be explained by the following.
On the discharge display panel having the construction as shown in FIG. 11, both the anodes and cathodes are arranged in parallel respectively and facing each other at short distance. Namely the discharge cells are arranged in a matrix. Each cell is formed by a cathode and an anode. The equivalent circuit diagram of this panel is considered to be as shown in FIG. 13 considering the capacities between the electrodes and the inductances in the electrodes. FIG. 13 shows a case having two rows and two columns. In this figure, capacitance C.sub.A between two adjacent display anodes, capacitance C.sub.K between two adjacent cathodes, capacitance C.sub.O between a display anode and a cathode, resistance R.sub.A and inductance L.sub.A of the anode and resistance R.sub.K and inductance L.sub.K of the cathode have been considered. These are defined as panel circuit parameters.
When the conventional sustaining pulses are applied to this circuit, oscillation is produced in the waveform by the above panel circuit parameters, together with resistance, capacitance and inductance components of the driving circuit system. Since the position in the panel is different for each of the discharge cells, the circuit parameters except capacitance will vary for each of the discharge cells and thus the amplitude of the oscillation or the like will vary depending on the location of the cells in the panel. At a cell having large oscillation amplitude, an abnormally high voltage is produced by the applied pulses which might lead to an erroneous discharge. As a result of this phenomena, a particular region of the panel tends to cause an erroneous discharge and rise-up time of the discharge becomes shorter.
As the method for decreasing such kind of oscillation, it has been considered to vary the circuit parameters. As stated above, in order to prevent an increase of oscillation amplitude of voltage producing in a particular region of the panel, it is required to eliminate any difference of the circuit parameters for all the locations. Thus the difference or deviation of the circuit parameters including the driving circuit system should be made extremely small. However, it is impossible to vary the circuit parameters of the discharge display panel substantially in view of its construction. Furthermore, the parameters other than the capacitance C.sub.O vary greatly when the size of the discharge display panel becomes larger. Namely, the parameters are smaller at an area near the driving end and larger at the side opposite to the driving end. From this, it is expected that at somewhere in the panel, an oscillation may tend to occur.
As has been explained above, in the conventional driving method, the waveform of the sustaining pulse has a shape of a single pulse having a very steep rise-up portion which varies from zero potential to a potential V.sub.SP very rapidly. By this reason, the sustaining pulse waveform will have an oscillation by the components of resistance in the discharge panel, and capacitance, and inductance thereof and hence an excess voltage is induced between the display anodes and cathodes so that erroneous discharges tend to occur. According to an increase of the panel size, the circuit parameters of the discharge display panel may vary extensively and sustaining pulse margin can hardly be obtained in such a large size panel. Here the sustaining pulse margin is defined by the following equation: EQU (V.sub.SP)max-(V.sub.SP)min
wherein,
When this value is positive, there exists a margin. If it is negative there is no margin at all and no memory operation is effected.
On the other hand when a television picture or the like is to be displayed, the writing-in period is restricted as mentioned above. By this reason, the width of the sustaining pulse including the rise-up time can not be so long. Furthermore, since one each of such driving circuit is needed for each one electrode, the total number of the driving circuits correspond with the number of electrodes. This means that the driving circuit better be made in a simple construction and for the driving voltage to be kept as low as possible. Since the number of the electrodes increases as the panel becomes larger, the above requirement is more stringent for a large size display panel. As mentioned above, for driving a large size panel, the pulse width of the sustaining pulse is delimited and the durable voltage is also limited in view of the circuit construction so that the above decrease of the sustaining pulse margin should be dealt with.